This invention will be discussed in relation to endoluminally deployed stent grafts but the invention is not so limited and can also be applied to the grafts for the human or animal body and where biological fixation is a desired or necessary function.
Throughout this specification, when discussing the application of this invention to the aorta or other blood vessels, the term “distal” with respect to an abdominal device is intended to refer to a location that is, or a portion of the device that when implanted is, further downstream with respect to blood flow; the term “distally” means in the direction of blood flow or further downstream. The term “proximal” is intended to refer to a location that is, or a portion of the device that when implanted is, further upstream with respect to blood flow; the term “proximally” means in the direction opposite to the direction of blood flow or further upstream.
The functional vessels of human and animal bodies, such as blood vessels and ducts, occasionally weaken or even rupture. For example, the aortic wall can weaken, resulting in an aneurysm. Upon further exposure to hemodynamic forces, such an aneurysm can rupture. One study showed that in Western European and Australian men who are between 60 and 75 years of age, aortic aneurysms greater than 29 mm in diameter are found in 6.9% of the population, and those greater than 40 mm are present in 1.8% of the population.
One intervention for weakened, aneurysmal or ruptured vessels is the use of an endoluminal device or prosthesis such as a stent graft to provide some or all of the functionality of the original, healthy vessel and/or preserve any remaining vascular integrity by replacing a length of the existing vessel wall that contains the site of vessel weakness or failure. Stent grafts for endoluminal deployment are generally formed from a tube of a biocompatible material and one or more stents to maintain a lumen therethrough. Stent grafts effectively exclude the aneurysm by sealing both proximally and distally to the aneurysm, and shunting blood through its length. A device of this type can, for example, treat various arterial aneurysms, including those in the thoracic aorta, abdominal aorta, iliac, or hypogastric artery.
Two closely related aspects of stent graft function are sealing and fixation. The stent graft typically engages the wall of the lumen on both ends of the aneurysm or other defect, at proximal and distal regions referred to as landing or sealing zones. The sealing zones are typically near the termini of the stent grafts. The seal between the stent graft and the vascular wall is typically formed at these locations as a result of the circumferential apposition of the stent graft to the vascular wall. Apposition is typically maintained by the radial force exerted by stents fixed to the stent graft.
It is also desirable to fix, or anchor, the stent graft in place. For some abdominal aortic aneurysm stent grafts, proximal fixation in the neck region of the aorta is critical for long term durability of endoluminal repair. Fixation of the stent graft in part depends on mechanical anchoring mechanisms. One anchoring mechanism, the frictional forces between the stent graft and aortic wall, may be created by the interference fit between the stent graft and aorta wall. The frictional forces may be supported by an underlying stent or stents. The practice of over-sizing a device for the lumen into which it is to be placed may also increase these frictional forces. Fixation may also be assisted by small hooks or barbs that extend from the stent graft and completely penetrate the arterial wall. In both cases, fixation is immediate and does not require long term biological interaction. In contrast, tissue encapsulation may also occur in some devices over a longer time frame. Exposed stainless steel stent struts and other parts of the stent graft may eventually become completely encapsulated by tissue growth, thereby assisting fixation.
The bifurcated stent graft, one example of an endoluminal device, is known for use in treating abdominal aortic aneurysms, where the stent graft at the proximal end defines a single lumen for placement within the aorta and at the other end bifurcates into the iliac arteries. One such stent graft, disclosed in PCT application WO98/53761 is useful for repair of abdominal aortic aneurysms. That application discloses a stent graft that includes a sleeve or tube of biocompatible graft material such as woven polyester fabric or polytetrafluoroethylene (PTFE) defining a main lumen and two iliac limbs. The stent graft further includes several stents secured therealong. The stent graft is designed to span an aneurysm that extends along the aorta between the iliac and renal arteries.
In the WO98/53761 application, the fabric-covered portion of the single-lumen proximal end of the stent graft bears against the wall of the aorta above the aneurysm and distal to the renal arteries to seal off the aneurysm. Thin wire struts of a juxtarenal attachment stent traverse the renal artery ostia without occluding them. Barbs on the attachment stent help anchor the stent graft in place.
An extension module is attached to one of the limbs of the stent graft to extend through a respective iliac artery and, optionally, an iliac extension module can be connected to the other leg. The deployment of a modular stent graft into the lumen of a patient from a remote location by the use of a deployment device or introducer assembly is disclosed in the same patent application. PCT application WO98/53761 is incorporated herein by reference.
One stent graft approved by the Food and Drug Administration (FDA) to treat aortic aneurysms is the ZENITH® AAA Endovascular Graft (Cook Incorporated, Bloomington, Ind.). The ZENITH® AAA Endovascular Graft is made up of three prosthetic modules: a bifurcated main body module and two leg modules. The main body is positioned in the aorta. The legs are positioned in the iliac arteries and connect to the main body. The stent graft thus extends from a section of the aorta, typically below the renal arteries and into both iliac arteries. The graft material is made of a woven polyester fabric like that used in open surgical repair. Standard surgical suturing techniques are used to sew the graft material to a frame of stainless steel stents. These self-expanding stents provide support for the graft material.
Through the physiological process of arterial disease and aneurysm growth, both fixation and sealing may become compromised. For example, the neck of the aorta can further dilate due to disease, normal aging or the outward force of the stent. A stent graft may thereby lose its seal, thus allowing an endoleak to form. Naturally, endoleaks can be a serious problem, preventing the prosthesis from performing its function. An endoleak, even a relatively small one, may result in the aneurysm repressurizing, increasing the risk of rupture. Loss of the apposition and interference fit may also reduce the frictional forces that keep the stent graft in place. Furthermore, the fixation provided by barbs can be under risk due to the failure of the fixation barbs or local tearing of the aortic wall. Loss of fixation is highly undesirable and may also contribute to patient mortality and morbidity. For some stent grafts, loss of fixation and resulting migration may result in the occlusion of one or more blood vessels. Stent graft migration itself can cause type I endoleaks and increase the necessity for surgical intervention to repair the endoleaks.